1. Field of the Invention
The present invention relates to an ink jet printing apparatus. Specifically, the present invention relates to a configuration of a cleaning device for cleaning an ink jet head (hereinafter referred to as a “printing head” or simply as a “head”) used for the ink jet printing apparatus. More specifically, the present invention relates to a configuration of a reservoir of a liquid for the head, the liquid being used for cleaning the head by removing an ink residue and the like which adhere to a face (hereinafter also referred to as an “ejection face”) of the printing head, and ink ejection openings being formed in the face.
2. Description of the Related Art
An ink jet printing method is a system for converting inputted image data to an outputted image by means of inks which are liquids. As a result, techniques for cleaning a printing head from which inks are ejected are very important for the inkjet printing method. Problems which need to be solved by cleaning the printing head will be explained as follows.
A printing head for ejecting inks ejects inks directly to a printing medium from its fine nozzles (hereinafter referred to collectively as ejection openings, liquid passages connecting with the ejection openings, elements for generating energy to be used for ejecting the inks unless otherwise specifically indicated). Accordingly, it is likely that inks ejected therefrom may bounce back after hitting the printing medium. In addition, it is likely that, while inks are being ejected, fine droplets (satellites) of inks other than main inks used for the print may be ejected therefrom to float in the atmosphere. Furthermore, it is likely that, if the fine droplets of inks float in the atmosphere, the fine droplets of inks may turn into ink mists to adhere to neighborhoods of the ink ejection openings of the printing head. Moreover, it is likely that dust floating in the air may be attached to the ejection openings of the printing head. As well, it is likely that, if the floating dust is attached to the ejection openings, the attachment may attract the main ink droplets ejected therefrom to deviate directions in which the inks are ejected from the should-be directions, that is, to hinder the main ink droplets from being ejected straightforward.
As one of cleaning techniques for solving these problems, what is termed as a wiping technique is adopted for the ink jet printing apparatus. In accordance with the wiping technique, the ejection face of the printing head is wiped by use of a wiping member (wiper) made of an elastic material such as rubber at a predetermined timing, and thus the attachment is wiped off the ejection face. The wiping technique of this type is also used for the following case.
It is likely that the ejection openings may be clogged by increase in the viscosities of the inks, solidification and deposition of the inks in the ejection openings, and the like, which result from dry of the inks in the vicinity of the ejection openings of the printing head. Furthermore, the ejection openings are clogged with bubbles generated inside the ejection openings (liquid passages), dust which intrudes in the ejection openings, or the like. As one of methods of preventing and solving this type of clogging, for example, a suction recovery method may be adopted. In accordance with the suction recovery method, an airtight system is formed of a capping member in the ink ejecting portion, and thus a suction force with a predetermined level of negative pressure is generated in the ejection face by use of a pump. Thereby, inks are forcibly discharged from the ejection openings. It is likely that inks may be attached to the ejection face in conjunction with the suction recovery method of this type. For this reason, the wiping operation is performed in order to remove the attachment.
Recently, instead of inks containing dye component (dye ink) as coloring materials, inks containing pigment components (pigmented inks) are increasingly used for the purpose of enhancing the printing density, water resistance, light resistance and the like of printed materials. Pigmented inks are produced through dispersing coloring materials, which are originally solids, into water by adding dispersants thereto, or by introducing functional groups to pigment surfaces. In the case of pigmented inks used at present, grain sizes of pigments are approximately 100 nm, and they are remarkably larger than sizes of dye molecules. For this reason, even if the pigmented inks are affected by light or ozone, color degradation of the pigmented inks is not obvious. The pigmented inks are far better in resistance to climatic conditions than the dye inks.
However, dried matter of pigmented inks which is produced through evaporation of water contents contained in the inks on the ejection face damages the ejection face more than attached matter produced through desiccation of dye inks in which the coloring materials themselves are dissolved at molecular level. In addition, high polymers used for dispersing the pigments into the solvent are apt to adhere to the ejection face. This type of adhesion is a problem which occurs in inks other than the pigmented inks in a case where high polymers exist in the inks as a result of adding a reaction liquid in the inks for the purpose of controlling the viscosities of the inks, for the purpose of enhancing the light resistances of the inks, or for another purpose. Moreover, the viscosities of the pigmented inks increase faster than the viscosities of the dye inks, and the pigmented inks adhere to the ejection face faster than the dye inks. As a result, the viscosities of the pigmented inks increase earlier than the viscosities of the dye inks, and the pigmented inks adhere to the ejection face earlier than the dye inks.
Accordingly, wiping performance exhibited by the scraping (or wiping) the adhered pigmented inks off the ejection face when the pigmented inks are used is poorer than wiping performance exhibited by the scraping (or wiping) the adhered dye inks off the ejection face. In other words, even if the wiping operation is performed, the inks still remain deposited in the form of a film on the ejection face, and the inks are hardened. As a result, the wiping operation can not realize an intended cleaning condition. Otherwise, it is very difficult to realize the intended cleaning condition through the wiping operation.
In the case of the dye inks, generally, dye molecules themselves are dispersed (dissolved) in the aqueous solution. In the case of the pigmented inks, however, pigment grains are generally not hydrophilic but hydrophobic. Accordingly, the pigment grains are not dissolved in the aqueous solution. For this reason, in order to make the pigmented inks water-soluble, a resin, an active agent or the like is adhered to the pigment grains. As a result, the pigmented inks are made hydrophilic as pigment dispersants, and thus the pigments are dispersed in the aqueous solution. Alternatively, hydrophilic groups are imparted to extremities of the structure of each of the pigment grains, and thus the pigment grains are self-dispersed in the aqueous solution.
Because the pigment grains themselves are hydrophobic, the pigmented inks have a tendency to make the ejection face wet unevenly when the pigmented inks are ejected from the printing heads, in comparison with the dye inks. So-called resin-dispersed pigmented inks, which are obtained by dispersing the pigments with the aforementioned resin, have a more obvious tendency to make the ejection face wet unevenly, because not only the pigments but also the resin is apt to make the ejection face wet. In addition, if the foregoing wiping operation is performed while the pigment grains exist on the ejection face, pigment aggregates are removed, and the ejection face is rubbed with the removed pigment aggregates. It is likely that the scratching of the top surface of the ejection face may change the surface characteristics of the ejection face. These hinder the ejection characteristics of the inks, that is, the stability in the ejecting direction. Accordingly, these decrease the accuracy with which ink droplets are landed at should-be positions. As a result, these may be a cause of deteriorating the image quality.
A countermeasure against these problems is to treat the ejection face of the printing head with a so-called water repellent finish If such a treated printing head is used, inks are ejected in the should-be directions stably at the beginning. However, in the case where inks, such as pigmented inks, which are apt to make the ejection face wet, is used, basically, the water repellency is gradually deteriorated, and accordingly the ejection characteristics become unstable gradually. In addition, the wiping operation also results in spread of the pigmented inks, which are apt to make the ejection face wet, throughout the ejection face, hence deteriorating the water repellency. Finally, the image quality is deteriorated.
By contrast, the following printing head has been proposed as a printing head for pigmented inks. In the case of this printing head, only vicinities of the ejection openings are made hydrophilic from the beginning so that the uneven wetness is corrected, as disclosed in Japanese Patent Laid-open No. 11-334074 (1999). However, this hydrophilic nature and the like can not be maintained for a long period of time, and are deteriorated with lapse of time. Even in a case where vicinities of the ejection openings are made hydrophilic by means of a UV ozone treatment or the like, as described in Japanese Patent Laid-open No. 11-334074 (1999), it is likely that the degree of the hydrophilic nature changes with lapse of time although the hydrophilic mature is maintained immediately after the treatment.
It is known that a so-called wet wiping technique as disclosed, for example, in Japanese Patent Laid-open No. 10-138502 (1998) is adopted to deal with the foregoing problems of change in water-repellent performance and hydrophilic performance of the ejection face. In the case of this technique, a liquid for the head (hereinafter referred to as a “wetting liquid”) made of a very low volatile solvent, such as glycerin and polyethylene glycol, is applied to the wiper for wiping the ejection face. By wiping the ejection face with the wiper to which the wetting liquid is applied, change in wettability of the ejection face is intended to be prevented. First of all, functions of the wetting liquid include an effect that the wetting liquid dissolves viscous matter and filmed matter of the inks accumulated on the ejection face. Second, the functions include another effect that the wetting liquid reduces abrasion of the wiper through working as a lubricant by causing the wetting liquid to interpose between the wiper and the ejection face. Third, the functions include yet another effect that the wetting liquid forms a film for protecting the ejection face by applying the wetting liquid to the ejection face.
A configuration in which a wetting liquid used for the wiping operation is stored inside the printing apparatus is adopted. In addition, the wetting liquid is intended to be stored inside the main body of the printing apparatus for a long period of time (for example, the life time of the printing apparatus ends). For this reason, it is desirable that a wetting liquid with a lower saturated vapor pressure in the air, or a wetting liquid hard to evaporate, should be used. Moreover, when the wetting liquid's capability of dissolving the viscous matter of the inks and the wetting liquid's quality of contacting each of the members constituting the head, are taken into consideration, it is desirable that a polyalcohol type solvent such as glycerin, which is often used as a composition of inks as liquids for printing should be used. Generally, many solvents of this type have a larger molecular weight and a higher viscosity. Accordingly, a rate at which the viscosity rises under a low-temperature environment is large in the case of those solvents. Although the viscosity of glycerin is, for example, approximately 800 cp at normal temperature, the viscosity is 2300 cp at 15° C., and 7000 cp at 5° C. As the temperature decreases, the viscosity increases in an accelerating manner. For this reason, if the printing apparatus is designed without paying attention to the glycerin supplying path from a section for holding the wetting liquid to a section for applying the wetting liquid to the wiper, a sufficient amount of the wetting liquid can not be applied to the wiper. As a result, the expected effect of the wet wiping operation can not be brought about.
On the other hand, glycerin has a property of absorbing moisture under a humid environment and thus expanding to a large extent. For this reason, the wetting liquid holding section with a capacity sufficiently large for holding glycerin which absorbs moisture, or an aqueous glycerol solution needs to be prepared in the main body of the printing apparatus. Supposed that, for example, approximately one mg of the wetting liquid needs to be applied to the wiper for each wiping operation, and that the wiping operation is performed ten thousand times before the life time of the printing apparatus ends, at least 10 g of glycerin needs to be stored in the printing apparatus. The 10 g of glycerin is obtained by multiplying one mg of glycerin for each wiping operation by ten thousand wiping operations. In addition to this, the following factors have to be taken into consideration: variations among printing apparatuses, use conditions different from one another depending on users, and use of the printing apparatus under the aforementioned humid environment. As a result, the amount of glycerin to be filled in the printing apparatus in the initial phase needs to be an amount obtained by multiplying the above-described minimum amount of glycerin by a safety factor. If, for example, the safety factor is 1.2, the amount of glycerin in the initial phase is 12 g. In addition, supposed that the volume of glycerin expands twice to four times by absorbing moisture under a humid environment, it is extremely desirable that a wetting liquid holding section with a capacity several times as large as the amount of glycerin filled in the initial phase should be used, and that a measure should be taken for the wetting liquid holding section not to allow the wetting liquid to leak even though the volume expands.
In this respect, it is conceivable that the wetting liquid holding section is made airtight in order for the ratio of components in the wetting liquid not to change due to an environment where the printing apparatus is used, or in order for the volume of the wetting liquid to change due to the environment. However, it is difficult to make the wetting liquid holding section completely airtight. In addition, a complicated mechanism is needed for making the wetting liquid holding section airtight.
On the other hand, it is conceivable that the wetting liquid is impregnated and held in a wetting liquid holding member made of a fibrous member having an adequate surface tension and an adequate size, and that such impregnation and holding prevent the wetting liquid from leaking when the volume of the wetting liquid expands. In addition, it is conceivable that a system in which the wetting liquid impregnated and held in the member is supplied to a member for applying the wetting liquid to the wiper is configured. In this case, the configuration is simple, but the system is susceptible to conditions changing under various environments. In a case where conditions in which the wetting liquid is impregnated and held in the wetting liquid holding member change due to moisture absorption, evaporation, change in amount of remaining wetting liquid, and the like, it is difficult to maintain performance of supplying the wetting liquid thereto stably and sufficiently.